Laterite ores are potentially the world's largest source of nickel and cobalt. In general, most deposits of nickel/cobalt laterites contain three major zones based on morphology, mineralogy and chemical composition. These three zones, from the base to the surface, atop weathered parent bedrock materials are the saprolite zone, the transition zone and the limonite zone. There is generally a large variation in total thickness of the laterite deposit, as well as individual zone thickness.
The saprolite zone consists predominantly of “saprolitic serpentine” minerals and a large variety of nickel/magnesium silicate minerals. The weathering process, or “serpentinization” of the parent bedrock material is characterised by a decrease in the magnesium content and an increase in the iron content of the top layer of ore body. The resulting saprolite zone contains between 0.5% and 4% nickel and a higher magnesium content, which is normally over 6% wt. Saprolite contains less cobalt. The Co/Ni weight ratio of saprolite is normally less than 1:10.
The not well defined transition zone is composed essentially of limonite and saprolite. It also commonly contains nickel in the range of from 1.0% to 3.0% with co-existing cobalt ranging from 0.08% up to 1%. Cobalt is generally associated with asbolane, a hydrated manganese oxide.
The limonite zone, located on the top zone of lateritic ore body, contains nickel ranging from about 0.5% to 1.8% and consists of goethite-rich and/or hematite rich ore, which is rich in iron and cobalt. Therefore the cobalt value of a lateritic ore body is mostly recovered from limonitic and transition zone. It has a lower magnesium content than saprolitic type ore. Due to stronger weathering, limonitic ore contains dominantly fine and soft particles of goethite and/or hematite. Sometimes the weathering has not been fully completed and either the hematite or the goethite rich sections are not present. Alternatively, depending upon the climatic condition, the limonite zone will still contain residual iron/aluminium silicates, such as nickel-containing smectite, nontronite and chlorite.
It has been found that the permeability of lateritic ore is largely controlled by the type of mineral occurrence, mineral morphology and particle size. Although the mineralogy of lateritic ore is rather complex and widely variable from deposit to deposit, there is some commonality or similarity of mineral morphology in the worldwide lateritic nickel deposits. These morphological structures enhance permeability of solution and preserve physical stability of individual minerals.
Heap leaching of nickeliferous oxidic ore has been proposed in recovery processes for nickel and cobalt and is described, for example in U.S. Pat. Nos. 5,571,308 and 6,312,500, both in the name of BHP Minerals International Inc.
U.S. Pat. No. 5,571,308 describes a process for heap leaching of high magnesium containing laterite ore such as saprolite. The patent points out that the fine saprolite exhibits poor permeability, and as a solution to this, pelletisation or agglomeration of the ore is necessary to ensure distribution of the leach solution through the heap.
U.S. Pat. No. 6,312,500 also describes a process for heap leaching of laterites to recover nickel, which is particularly effective for ores that have a significant clay component (greater than 10% by weight). This process includes sizing of the ore where necessary, forming pellets by contacting the ore with a lixiviant, and agglomerating. The pellets are formed into a heap and leached with sulfuric acid to extract the metal values. Sulfuric acid fortified seawater may be used as the leach solution.
International application PCT/AU2006/000606 (in the name of BHP Billiton SSM Technology Pty Ltd) also describes a process where nickeliferous oxidic ore is heap leached using an acid supplemented hypersaline water as the lixiviant with a total dissolved solids concentration greater than 30 g/L in order to leach the heap.
Heap leaching laterites offers the promise of a low capital cost process, eliminating the need for expensive and high maintenance, high pressure equipment required for conventional high pressure acid leach processes.
Generally, in a heap leach process of laterites, a relatively strong acidic lixiviant is used to liberate both the cobalt and nickel from the cobalt and nickel containing ores. With laterite ores predominantly consisting of saprolitic type ores, the nickel is generally associated with saprolitic serpentine minerals and nickel/magnesium silicate minerals. In limonitic ore the nickel is generally associated with the iron containing minerals such as goethite. Cobalt is commonly associated with hydrous manganese oxides, such as asbolane, in both saprolitic and limonitic ore. Generally, in heap leaching processes, the lixiviant is a relatively high strength acidic solution that liberates both the nickel and cobalt from the respective minerals within the laterite ore.
The leachate will generally include dissolved nickel and cobalt together with other dissolved impurities and in order to recover the nickel and cobalt, separation techniques such as sulfide or hydroxide precipitation, solvent extraction or ion exchange need to be employed to separate the nickel and cobalt. The present invention aims to develop a heap leach process where the nickel and cobalt can be at least partially separated as part of the leaching process to obtain the PLS (pregnant leachate solution) with higher cobalt/nickel concentration ratio than the cobalt/nickel concentration ratio of ore, which obviates or reduces the need for downstream separation techniques.
A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.